In hand-held portable motor-driven saws, used for removing branches from trees for instance, high-speed operation at approximately 9,000 to 12,000 rpm is typical. On the other hand, in felling trees, rotational speeds of approximately 6,000 to 9,000 rpm are used for maximum cutting power. The idling rotational speed is approximately 2,000 to 3,000 rpm. German published and examined patent application DE-AS No. 22 48 584 discloses a two-stroke engine in which the fuel is aspirated by the injection pump with increasing pressure in the crankcase and then, as the crankcase pressure increases further, the fuel is pumped so as to be injected into the combustion chamber. However, a satisfactory adaptation of the instant of injection to the particular rpm is not obtained. The injection event itself, that is, injection of the fuel into the combustion chamber of the engine, should preferably take place approximately when the piston is in the vicinity of bottom dead center. At relatively high engine speeds, however, there are delays because it takes a certain amount of time for the air pressure to reach the injection pump from the crankcase. The movement of the pump piston in the injection pump takes still more time. Even further delay is occasioned by the distance the fuel is pumped.
At high rpm, these delays are so major that even though the injection event was triggered in the vicinity of bottom dead center, the actual onset of injection occurs only just before top dead center of the piston. The end of the injection event is thus delayed still further, until the end of injection extends past top dead center and occurs while combustion is already taking place, resulting in poor efficiency. These delays occurring at high speeds mean that optimal combustion no longer takes place; individual combustion events may be entirely absent and the overall operating cycle of the two-stroke engine at various speeds is disrupted.